The invention is in the field of electronic video technology and is directed to a method for image analysis and correction of video signal values in the processing of video films.
In the production of a video film, individual scenes are recorded with a video camera, whereby each scene represents a video sequence composed of a plurality of frames, and the video signal values are deposited in an image store. After the shots, the video film is processed in a video processing station. In the post-processing, the design of the video film occurs by cutting and by producing titles as well as a color correction supported by an image analysis with which deficiencies are compensated or colors are editorially changed.
In an image analysis, color values are investigated, for example, in view of the image range, the image gradation and a potentially existing color cast. Image light and image dark values are acquired with an image range analysis for an adaptation of the image range; a gradation correction curve for a contrast correction is determined with an image gradation analysis; and color cast values for a color cast correction are determined with a color cast analysis.
For the traditional color correction, analog image analysis methods based on the color values R, G and B are usually employed wherein the image range analysis and the color cast analysis occur directly on the basis of the color values R, G and B, and the image range analysis occurs on the basis of a brightness signal derived from color values R, G and B. The traditional methods have the disadvantage that they are relatively slow and that the analysis results must be largely manually converted into corresponding corrections.
DE-C-43 10 727 already discloses a method for the analysis of an image original to be reproduced in the preliminary printing stage, whereby RGB color values are transformed into corresponding Lab color values of a sensation-conforming, equidistant CIELAB color space, and the analysis of the image original is undertaken on the basis of the transformed Lab color values, as a result whereof optimum analysis results are nearly automatically obtained in view of image range, image gradation and/or color cast. This publication, however, contains no teaching to apply the analysis method to the video signal values of individual frames and video sequences of a video film.
EP-A-0 723 363 discloses a method for realizing an inventive color adaptation, whereby RGB color values of individual color images or color image sequences are transformed into a color space adapted to the visual sensation of man and are subsequently transformed back. Although the known method is applied to color image sequences, no analysis with respect to image range, image gradation and/or color cast occurs on the basis of transformed Lab color values of the CIELAB color space.
It is therefore an object of the present invention to improve a method for image analysis and correction of video signal values in the processing of video films such that optimum analysis results are achieved for a color correction of video frames and video sequences.
According to the present invention, a method is provided for image analysis and correction of video signal values and processing of a video film composed of video sequences, each video sequence being composed of a plurality of frames. Video signal values of a first color space are transformed according to a three-dimensional conversion function into video signal values of a second color space that is independent of the first color space and is equidistant and conforming to sensation. At least one analysis value representative of a video sequence is formed by forming an average of analysis values of analyzed frames of the video sequence. A three-dimensional correction function is respectively determined from the analysis values representative of the video sequence. At least the video signal values of the corresponding video sequence are corrected according to the identified correction function.
The invention is explained in greater detail below on the basis of the FIGURE.